1. Field of the Invention
The present invention relates to portable devices for performing blood coagulation tests on blood such as a prothrombin time tests, and the associated methods used to preform such tests.
2. Prior Art Statement
Many people with heart disease, venous thrombosis, history of strokes and the like are prescribed medications to reduce the coagulation characteristic of their blood. A commonly prescribed medication is sodium warfarin isopropanol clathrate, generically known as warfarin and commonly known by the brand name COUMADIN.RTM., produced by DuPont Pharmaceuticals of Wilmington, Del. Warfarin acts to inhibit the synthesis of vitamin K dependent factors. The resultant effect in the body of the patient is a sequential depression of blood Factors VII, IX, X and II. Upon the oral administration of warfarin, the anticoagulation effect generally occurs within 24 hours. However, depending upon the patient, the peak anticoagulant effect may be delayed 72 to 96 hours and its duration may persist for four to five days. Accordingly, it is known that warfarin is a potent drug with a half-life of approximately 21/2 days, therefore the effects of warfarin on a patient may become more pronounced as daily maintenance doses overlap. Accordingly, due to the potency of warfarin and its long lasting effects, it is critical that the level of warfarin in a patient be closely monitored. An overdose may cause spontaneous internal bleeding while an underdose would not be an effective anticoagulant, resulting in thrombosis. Additionally, the need for exact blood monitoring is more critical with warfarin than with many other drugs because the relative effectiveness of warfarin is affected by many other prescription and over-the-counter medications that may be taken by a patient using warfarin.
The most common manner of determining the effective amount of warfarin in a patient's blood is to preform a prothrombin time (PT) test. A PT test measures how long a sample of blood takes to clot. As a result, the amount of anticoagulant in the blood can be calculated since the concentration of anticoagulant is directly proportional to the length of time required to form clots the blood sample.
The prior art is replete with various devices and methods for measuring the coagulation time of blood samples. For example, such a method and apparatus is shown in U.S. Pat. No. 5,302,348 to Cusack et al., entitled BLOOD COAGULATION TIME TEST APPARATUS AND METHOD, which issued Apr. 12, 1994 and is assigned to International Technidyne Corporation, the assignee herein. This patent shows a machine that measures the coagulation time of blood placed in disposable cuvettes by pneumatically moving the blood through a single restricted passage in the cuvette.
U.S. Pat. No. 5,154,082 to Michael Mintz, entitled MICROPROCESSOR-CONTROLLED APPARATUS AND METHOD FOR DETECTING THE COAGULATION OF BLOOD, issued Oct. 13, 1992 and assigned to the assignee herein, shows a device that measures blood coagulation by its effect on a ferromagnetic member in an electromagnetic field.
U.S. Pat. No. 4,797,369 which issued on Jan. 10, 1989 entitled METHOD AND APPARATUS FOR DETECTING A BLOOD CLOT to Michael Mintz, and assigned to the assignee herein, shows the technique for measuring clot time whereby a sample of whole blood or blood plasma is dispersed into two or more zones. The zones are separated and brought together repeatedly, such that the blood sample is divided into multiple pans each associated with a zone. The pans are then rejoined into a single pan and the process of separation and joining continues. During the process, a liquid bridge between the separated parties is initially supported by surface tension, but collapses at the point of maximum zonal separation. When a fibrin clot is entrained within the rejoined parts, it will align in a direction parallel to the direction of the relative motion between the zones. In this manner, a thread appears between the pans as they are being separated. This thread is indicative of a clot, wherein the which clot is capable of being detected by visual or electrical means.
U.S. Pat. No. 3,486,859 entitled BLOOD ANALYZING METHOD AND APPARATUS issued on Dec. 30, 1969 to R. Greiner et al. This patent depicts a blood analyzing method and apparatus including a double arm holder having blood liquid reactant chambers which communicate with each other via a small capillary conduit. An air pump is provided for applying pressure changes to one of the chambers to effect periodic mixing of the liquids via the capillary conduit. An indicator means is included to detect the progressive restriction of the capillary conduit upon coagulation of the blood.
U.S. Pat. No. 3,695,842 entitled METHOD AND SYSTEM FOR ANALYZING A LIQUID issued on Oct. 3, 1972 to M. D. Mintz, and assigned to the assignee herein. The patent describes in detail a magnetically coupled mechanical blood clot detection system wherein a variable conductance device is disposed adjacent to a zone containing a liquid and a member. A predetermined variance in the conductance of the device is detected upon change in magnetic flux lines when the liquid transforms itself and the member is displaced. The signal is produced at the time the predetermined variation in conductance has been detected.
An improved system means for measuring clotting time is disclosed in U.S. Pat. No. 3,836,333 entitled "SYSTEM FOR TIMING THE COAGULATION OF BLOOD" issued to Michael D. Mintz, on Oct. 30, 1972 and assigned to International Technidyne corporation, the assignee herein. An electromagnetic bias coil, which is wound around the reed switch, provides steady-state magnetic flux lines that supplement the flux lines provided by the permanent magnet. When the density of the flux lines passing through the reed switch decreases, as a result of the magnet being displaced, the reed switch opens. The bias coil also provides a magnetic pulse, which forces the reed switch to a closed state. This system is manufactured under the trademark HEMOCHRON.RTM. by International Technidyne Corporation at Edison, N.J.
U.S. Pat. No. 3,890,098 entitled MACHINE FOR THE DETERMINATION OF PROTHROMBIN TIME AND P.T.T. issued on Jun. 17, 1975 to E. Moreno. This patent describes a reactive material which is placed in a cup which communicates with a second cup via a restricted orifice. Plasma is placed in the second cup and the reactive material and plasma are moved from cup to cup by a pump until coagulation of the plasma takes place. Means are then provided for stopping the motion of the mixed reactive material and plasma. Other means are provided for measuring the time required for coagulation.
U.S. Pat. No. 3,951,606 entitled APPARATUS FOR PROTHROMBIN TESTING issued on Apr. 20, 1976 to R. Moyer et al. This patent shows a manually operable disposable device which can measure coagulation rates. The device is a tube of a uniform bore which can accommodate a sample and contains appropriate amounts of lyophilized reagents required to conduct individual tests such as that for prothrombin time. Calibration marks on the tube are correlated in terms of these times and the position in which a liquid sample becomes immobilized as it descends down the tube corresponds to the test time. The rate of descent of the liquid is controlled by a limiting orifice or constriction or by inclining the tube to the vertical axis.
U.S. Pat. No. 4,197,734 entitled APPARATUS FOR DETERMINING BLOOD CLOTTING TIME issued on Apr. 15, 1980 to A. Rosenberg. This patent describes an apparatus which is capable of determining the clotting time of blood. The apparatus includes a support frame which supports a syringe containing a blood sample and turntable adapted to rotate at a normal rate of speed. Blood from the syringe drops onto the turntable where the clotting time is automatically and graphically depicted by a chart rotatively carried upon the turntable. The apparatus can also be employed to determine variations in the viscosity of blood plasma and other fluids.
U.S. Pat. No. 4,725,554 entitled METHOD FOR MEASURING BLOOD COAGULATION TIME issued on Feb. 16, 1988 to K. Schildkenecht. This patent shows a method for measuring the coagulation time of a blood sample, in which a sample reagent mixture is formed by introducing the sample and at least one reagent into a cuvette. The sample reagent mixture is moved in a stationary cuvette so that the mixture flows back and forth around an edge projecting in to the cuvette whereby a clot forms and is detected on this edge.
U.S. Pat. No. 4,659,550 entitle METHOD AND APPARATUS FOR MEASURING BLOOD COAGULATION TIME is the parent of U.S. Pat. No. 4,725,554 and essentially describes the same system further utilizing photocell detectors to determine a clot formation.
From the above-cited patents, it can be seen that many devices have been made that are capable of performing a coagulation time test. However, most of these prior art devices are designed for use in a medical laboratory environment. Few of these devices are portable and simple enough to operate by a patient in his or her home. Furthermore, the prior art devices only provide the user with a single test result for the current blood sample. None of the cited references are capable of analyzing the current blood sample in view of previously stored results in order to provide a historical analysis and a means to have statistical process control over the long time use of warfarin.
One of the most difficult aspects of using the cited prior art test devices is the taking of the blood sample from the patient and the administration of the blood sample to the testing equipment. In many cases, the taking of blood requires a syringe or evacuated tube and the administration of the blood to the test equipment requires complex measurements and the use of pipettes. Obviously, such devices are not intended for unassisted home use.
A need therefore exits in the art for a portable, reliable and accurate self contained blood coagulation testing device that is simple to use and can be used at home by an unassisted patient.
The need also exists in the art for a blood coagulation testing device that has a simple means of donating the blood sample from the patient and properly administering the blood sample to the testing device.
A further need exists for a personal blood coagulation testing device that stores multiple readings from a single patient over a given period of time and presents a historical analysis of a current test.
These needs are provided for by the present invention as described and claimed below.